This invention relates to apparatus for fabricating molded plastic articles and for continuously obtaining plastic preforms in the shape of the hollow parts of a mold, which may produce a design or pattern on the articles. In particular, the invention relates to a new, automatic, molding apparatus having a turntable carrying a plurality of molds, which are subjected successively to heating and cooling.
It is well known that various molded articles, for example the parts composing boot uppers and legs made of synthetic material, can be obtained by filling the hollow parts of a mold with a plastic such as polyvinyl chloride in powder or plastisol form, the hollow parts being the negatives of the design to be reproduced. After application to the mold of a material forming the lining (in the case of a boot part), the mold and its contents are subjected to high-frequency heating, while the contents of the mold are placed under pressure, after which cooling enables the formed plastic part corresponding to the hollow mold design to be parted immediately from the mold. Such a technique, in several versions, is well known and has been described in a number of patents.
Known molding apparatus based on the aforementioned technique presently operate automatically. The most sophisticated of these known apparatus are of the carrousel type, which have a turntable on which the molds filled with plastic and closed under their mold-carrying plates are disposed at regular intervals, the molds being brought successively under a high-frequency heating press, then under a cooling press, which presses are mounted at fixed points around the turntable. Such apparatus indeed give satisfaction but their use is confined to fabrication of thin articles; in fact it is virtually impossible, while maintaining a profitable rate of manufacture, to cool the molded articles under pressure, and hence dents and other molding deformations cannot be avoided, especially when the article is not flat and has a raised pattern or other kinds of protrusions. Moreover, during operation it is difficult to avoid jolts of the machine which cause the pieces positioned in the molds to shift undesirably. Furthermore, it is difficult to obtain by means of the classic screw spindles used on presently known presses the extremely high compressive pressures often necessary. Finally, defects in parallelism of the molds are difficult to avoid, in addition to which high-frequency heating presses often interfere with telecommunications systems in the vicinity, radio and television sets, and the like.